The long-term objective of our research is to develop new drug delivery systems for macromolecular drugs that must function inside the target cell. The biotechnology and pharmaceutical industries have developed a wide variety of potential therapeutics based on the molecules of biology: DNA, RNA and proteins. For therapeutics such as plasmid DNA, antisense oligonucleotides, ribozymes, and immunotoxins, their ability to reach their target is dependent on their initial ability to reach the cytoplasm from the endosomal compartment. While these therapeutics have tremendous potential, effectively formulating and delivering them has also been a widely recognized challenge. There are a variety of difficult barriers, including drug stability, tissue penetration and transport. While a number of creative delivery systems show significant potential for overcoming these problems with biomolecules that act at the extracellular membrane, a widespread barrier for those that function intracellularly is cytoplasmic entry. Passive or receptor-mediated endocytosis results in localization of biomolecules to the endosomal compartment, where the predominant trafficking fate is fusion with lysosomes and subsequent degradation. Similarly, the delivery of plasmid-based or protein/peptide based molecular vaccines is also dependent on getting the plasmid to the nucleus, or the peptides or proteins into the cytoplasm for entry into the protein processing and display pathways. This proposal is aimed at developing synthetic polymeric carriers that mimic the highly efficient intracellular delivery systems found in nature, without the immunogenicity and dangerous properties of viral and pathogenic systems. Their most important property ties together the sensing of pH changes to membrane destabilizing activity. The "smart" polymer carriers thus enable endosomal escape and aid the transport of the macromolecular drugs to the cytoplasm, to circumvent the lysosomal trafficking fate. The carriers should enable the efficient delivery of a wide range of biotherapeutics, and open new families of drug candidates that attack intracellular targets.